Bauxite is the major source of aluminium containing ore used in the production of alumina. Bauxite contains hydrated forms of aluminium oxide (alumina) that occur in several different structural forms, depending upon the number of molecules of water of hydration and the crystalline form. Most commercially useful deposits of bauxite include gibbsite (alumina trihydrate) and/or boehmite (alumina monohydrate) and/or diaspore.
Alumina is extracted from bauxite by use of the Bayer process. Briefly, the Bayer process includes the steps of contacting bauxite with a hot caustic solution to dissolve alumina therefrom. If the bauxite contains mainly gibbsite, extraction of alumina from the bauxite may be conducted using a caustic solution at a temperature generally in the range of 100 to 150.degree. C. If the bauxite contains mainly boehmite, or diaspore higher temperatures, in the order of 200 to 300.degree. C. are generally required. For mixed bauxites containing both gibbsite and boehmite, a double digestion process may be used.
After digestion, the bauxite/caustic solution mixture is separated into a pregnant liquor containing dissolved alumina (usually in the form of sodium aluminate) and a solids residue (usually referred to as red mud). The pregnant liquor is fed to a precipitation circuit where it is cooled and seeded with solid particles of alumina trihydrate to induce precipitation of alumina trihydrate from the pregnant liquor. The resulting precipitation slurry is separated into a spent liquor stream and a solids stream. Coarse solids represent product and are transferred to a calcination stage where they are calcined to produce alumina. Fine solids are returned as seed particles to the precipitation circuit. The spent liquor is returned to the digestion step where it is contacted with further bauxite. Between the digestion and precipitation steps, there is generally one or more washing steps and the spent liquor generally must be evaporated to obtain the required caustic concentration prior to being returned to the digestion step. The Bayer process has been used commercially for about 100 years and it is well known to persons of skill in the art.
Bauxite, in addition to containing hydrated forms of alumina, includes several impurities. The main impurities are compounds of iron, titanium and silica. The compounds of iron and titanium found in bauxite generally are insoluble in caustic solutions and have little effect on the selective extraction of alumina from bauxite. These compounds report to the red mud following digestion.
The silicon compounds present in bauxite occur mainly as quartz and as hydrated double salts with alumina, such as kaolin. Quartz dissolves slowly in caustic solutions and the other forms of silica in the bauxite may dissolve rapidly in the caustic solutions used in the digestion step. Accordingly, bauxites containing significant amounts of silica have the potential to be difficult to treat.
The presence of silica in bauxite can cause at least two major problems in the digestion of bauxite, these being:
(i) dissolution and reprecipitation of silica as complex sodium alumino-silicates, thereby consuming caustic soda; and PA1 (ii) reprecipitation of complex sodium alumino-silicates on plant surfaces, thereby causing scale build-up. This problem is especially severe when scale builds up on heat exchange surfaces. PA1 the bulk of the silica is chemically combined with high usage of reagents such as lime or soda, and deported as solid alkali and alkaline earth silicates into a general mud residue (containing many other contaminants and having poor handling characteristics) from which chemical values cannot easily be recovered, or PA1 by use of very short contact times a limited fraction of the non quartz silica is deported unconverted to residue, thereby reducing by this fraction caustic consumption in the formation of sodium aluminosilicate reporting to mud residue, but only where this silica is contained in a gibbsitic bauxite or bauxite fraction, or PA1 a limited fraction of the silica is taken into solution, and after separation of liquor from mud residue, is precipitated from liquors as a clean aluminosilicate, facilitating the recovery of chemical values from aluminosilicate via subsequent techniques. PA1 (a) contacting the Bayer process feedstock with a caustic liquor under the following process conditions which result in dissolving and stabilising at least 50% of the silica into solution at a level of at least 3 gpL and without significant precipitation from solution of the dissolved silica: PA1 (b) separating the silica bearing liquor from the solid residue of silica dissolution step (a) under conditions which do not promote significant precipitation of the silica. PA1 1) For conventional predesilication processes, in which silica is deliberately fully converted to aluminosilicate desilication product the maximum dissolved silica levels are low (normally well less than 50% of the available silica is in solution at any point) and solution stability times are short. Therefore, the extraction of silica from bauxite is low (and subsequent solid/liquid separation is normally not practised). PA1 2) For low temperature digestion processes, aimed at avoiding some silica conversion to sodium bearing compounds and applicable only to gibbsitic bauxites, high dissolved silica levels are not maintained for any useful time. PA1 3) For double digestion processes, the problems associated with conventional predesilication and low temperature digestion are not avoided, and there is no other benefit which would assist in the treatment of high silica bauxites. PA1 4) For sinter processes, maximum dissolved silica levels are high, but still represent only a small fraction of total silica in bauxite. Also, the bauxite requires pretreatment (roasting with additives), and is generally thought to be uneconomic for future alumina developments.
Prior art attempts to deal with problems associated with silica in bauxite have concentrated on either suppressing silica dissolution or completing precipitation of silica in a controlled step to minimise scaling throughout the remainder of the plant. The problems of caustic soda consumption and scaling of plant surfaces are largely independent of each other--a low silica bauxite will result in low caustic soda losses but can drive significant scaling problems while a high silica bauxite will consume large quantities of caustic soda but will result in less scaling. It is for this reason that most of the prior art dealing with the impact of silica on the Bayer process deals with only one aspect of the problem. The prior art can be grouped broadly into four areas, as discussed below:
1) Predaesilication
A significant number of refineries include a so-called predesilication operation prior to digestion where the bauxite is held at a temperature of around 100.degree. C. for 6-18 hours. The purpose of this operation is to convert a large portion of the reactive silica to sodalite type sodium alumino-silicate which will then act as seed to rapidly convert the remaining reactive silica to sodalite type sodium alumino-silicate during digestion. The conditions under which predesilication is conducted, low caustic and alumina concentration, ensure that only a very small proportion of the total reactive silica is in solution at any given time. The primary purpose of predesilication is to ensure that conversion of reactive silica to sodalite type sodium alumino-silicate is complete so that pregnant liquor from digestion contains a minimum amount of dissolved silica which in turn minimises alumina product contamination and sodium alumino-silicate scaling during subsequent reheating of spent liquor. This operation has no impact on the amount of caustic soda consumed as a result of silica reaction. Sodium alumino-silicates are usually discarded from the Bayer plant as a component of red mud. However, a separate sodium alumino-silicate precipitation step after predesilication has been proposed.
U.S. Pat. No. 3,413,087 in the name of Roberts, assigned to Reynolds Metals Company, describes a Bayer process for extracting alumina from bauxite which includes a predesilication step to dissolve silica prior to digestion. In the predesilication step, the bauxite is mixed with spent liquor or strong liquor containing make-up caustic. The quantity of caustic present in the liquor is insufficient to dissolve all of the soluble alumina in the bauxite but is sufficient to dissolve substantially all of the soluble silica in the bauxite. However, only a small fraction of the soluble silica is in solution at any time. The slurry (of bauxite and liquor) is maintained in the predesilication stage (called a predigestion stage in the patent) at a temperature of from 150.degree. F. to the temperature used in the digestion step for a period of time (e.g. 30 minutes to 12 hours) to allow the dissolved silica to crystallize and precipitate as a complex sodium aluminium silicate desilication product. The patent states that crystallization of desilication product (DSP) causes the dissolved silica to preferentially precipitate on the DSP particles, rather than on other surfaces such as heat exchange surfaces. The turbulence of the slurry in the digestion system can also act to maintain clean heat exchange surfaces. The DSP is insoluble and allows the slurry to pass to the digestion stage without scaling of heat exchange surfaces occurring. After digestion, the DSP is removed in the red mud residues.
A paper by Eremin, from the USSR Institute of Mining, Leningrad, entitled "Beneficiation of Low Grade Bauxite by Hydrometallurgical Methods" (in: Proc. Conference; Alumina Production until 2000, Tihany, Hungary, Oct. 6-9, 1981, p. 135-142.) discloses studies into the dissolution of silica components from bauxite. Following these studies, the paper concluded that bauxite desilication should be carried out at approximately 80 to 90.degree. C. at a high liquid to solids ratio and with medium caustic concentrations (100 to 150 g/l Na.sub.2 O, which corresponds to 170 to 260 g/l, calculated as Na.sub.2 CO.sub.3). This paper makes reference to a treatment step in which a portion of the reactive silica is dissolved before Bayer process digestion. The reactive silica which enters solution is subsequently precipitated to produce a separable aluminosilicate material. However, this paper highlighted a major limitation in carrying out the process, since stable silica levels in solution never exceeded those expected in recycled Bayer liquors by more than about 2.5 gpL, limiting the effectiveness of silica dissolution at realistic liquor to bauxite ratios for bauxites having high silica contents. Thus in the best tests reported by Bremin only about 50% of the reactive silica was removed. For this reason Eremin proposed the use of a bauxite precalcination step to activate the silica and deactivate the alumina in the bauxite to a degree so that better selectivity could be obtained.
2) Low Temperature Digestion
A number of processes have been disclosed which aim to reduce dissolution of reactive silica. These processes are based on a low temperature digestion of bauxite where the very short residence time is just sufficient to extract alumina but insufficient to completely convert reactive silica to DSP. The temperatures required for alumina extraction is such that the desilication reaction still proceeds very rapidly so that it is not practically possible to achieve a solid-liquid separation for the purposes of substantial silica removal from the solid residue when the silica content of the liquor is at its highest. In order to minimise silica conversion to DSP or dissolution it is necessary to limit the digestion temperature to a maximum of 150.degree. C. meaning that this process is only suitable for gibbsitic bauxites. Only limited reduction in caustic consumption is achieved through reduced silica dissolution. Pregnant liquor must be separately desilicated after mud separation as it contains a high level of silica (typically 2-3 g/l).
U.S. Pat. No. 4,661,328 in the name of Grubbs, assigned to Aluminium Company of America, describes a process for purifying an alumina rich ore containing more than 5 weight percent reactive silica. The process includes the steps of mixing the ore with an aqueous digestion solution of silica and sodium aluminate and digesting the mixture at a temperature of 80 to 150.degree. C. to dissolve alumina whilst inhibiting the dissolution of reactive silica from the ore. The bauxite may be mixed with an aqueous digestion solution that is nearly saturated with silica and nearly saturated with alumina. In this process, the high silica ore is mixed with an aqueous digestion solution having high alumina, high silica and high soda in solution. The silica is present in the digestion solution at a concentration of greater than 1.8 g/l, typically 1.8 to 2.5 g/l. A post desilication process, seeded by desilication product, is required. Alumina may be present in the solution in an amount of from 150 to 170 g/l while soda values in the solution typically fall within the range of 240 to 300 g/l, calculated as Na.sub.2 CO.sub.3. The temperatures in the digestion step are lower than in conventional Bayer process temperatures and they typically fall within the range of 80 to 150.degree. C., most preferably 100 to 120.degree. C. This process retards or avoids the dissolution of silica from bauxite. The process can only be used with gibbsitic bauxite as the digestion temperatures utilised in this process are too low to viably digest boehmite containing bauxites.
U.S. Pat. No. 3,716,617 in the name of Oku et al, assigned to Sumitomo Chemical Co Limited, relates to a bauxite digestion process in which a digestion residue comprising solid components in a slurry that has not been subjected to a desilication treatment is separated by use of a synthetic organic high molecular weight flocculent from a sodium aluminate liquor resulting from bauxite digestion. The separation is conducted whilst at least 5% of the reactive silica remains unconverted to DSP in the solid residue. In the digestion step of this patent, the soda content may fall within the range of 80 to 200 g/l. The temperature is preferably 90 to 150.degree. C. The process is based upon operating the digestion step to reduce the amount of reactive silica dissolved from the bauxite. The combination of temperature and residence time in the process is chosen to minimise silica dissolution. Typical operating parameters for the digestion step include operating at a temperature of 110.degree. C. for ten minutes, or operating at 140.degree. C. for 60 seconds. The process described in Oku et al avoids complete silica dissolution or conversion to DSP during bauxite digestion.
Japanese Patent Application No. Sho 62-230613 discloses a process for extracting alumina from reactive bauxite in which the temperature and residence time are controlled to obtain substantially complete alumina dissolution, but only a small amount of the silica in the bauxite is extracted into solution or converted to DSP. After the required digestion time, the digestion slurry is rapidly cooled to quench the silica dissolution reaction. This process relies upon the differences in the rates of dissolution of alumina and silica to minimise the amount of silica extracted into solution or converted to DSP. Any silica that does go into solution is removed by seeding with sodalite to promote sodium alumino-silicate precipitation.
WO 93/20251 in the name of Sumitomo Chemical Company, Limited discloses a bauxite digestion process in which bauxite and an alkaline solution are mixed to form a slurry. The slurry is fed to an extractor and alumina is extracted whilst dissolution of reactive silica is suppressed. The solid residue is separated from the liquor without any of the reactive silica dissolved in the liquor being precipitated as a sodium alumino-silicate. The operating conditions in the extractor include an Na.sub.2 O content in solution of 100 to 160 g/l, a temperature of 110 to 160.degree. C. and a residence time of up to 10 minutes. As alumina is extracted from the bauxite, the liquor fed to the extractor must have a relatively low dissolved alumina content. In the extractor, the extraction of reactive silica does not exceed more than 70%, preferably not more than 5 by weight. The re d mud recovered from the digestion step contains a substantially reduced amount of sodium alumino-silicate. The liquor recovered from the digestion step contains only a limited amount of the original silica in the bauxite, and it is desilicated by adding a solid silicate material as seed to form insoluble silicate materials, such as sodalite or zeolite. Prevention of sodium alumino-silicate precipitation in the extractor is achieved by using a short residence time in the extractor.
U.S. Pat. No. 5,122,349 discloses a gibbsitic bauxite digestion process in which the gibbsite is rapidly dissolved to reduce the free hydroxide concentration. This leads to a reduced kaolinite dissolution. This process is only applicable to gibbsitic bauxites.
3) Double Digestion
Double digestion is a process designed for mixed, gibbsitic/boehmitic bauxites which allows boehmite to be extracted at a lower temperature than required in a normal high temperature plant. The effect of this is that dissolution of a portion of the silica in the form of quartz can be reduced. However, quartz usually only accounts for a small proportion of the total reactive silica in bauxite. Conversion of silica in kaolinite to DSP cannot be avoided in a double digestion process if the bauxite is handled as a single stream. However, in the unusual case that bauxite can be beneficiated to give gibbsite/kaolinite and boehmite fractions, caustic consumption can be reduced in the low temperature digestion. The separation of some silica (or sodium alumino-silicate) is possible after the low temperature digestion stage by keeping the temperature and residence time to a minimum and then desilicating the pregnant liquor after mud separation.
U.S. Pat. No. 4,994,244 in the name of Fulford et al, assigned to Alcan International Limited, relates mainly to a digestion process that separates red mud from liquor in a pressurised vessel. However, a side benefit from the process described in Fulford is that it is possible to limit the residence time of the bauxite in the digestion step to that time actually required for extraction of alumina from the bauxite. According to Fulford et al, conventional digestion processes carry out a significant part of the liquor desilication operation inside the pressure digester. As the liquor desilication reaction is relatively slow, the liquor desilication reaction typically controls the residence time required in the digester. With the process of Fulford et al, little or no liquor desilication occurs in the digester and a seeded, controlled desilication of the liquor can be carried out after the pressurised red mud separation step. This is, of course, a post desilication reaction. It is noted that one variant of the Fulford et al process (FIG. 6 in U.S. Pat. No. 4,994,244) utilises a double digestion step with desilication occurring in the digester used for gibbsitic digestion. The patent does not specify the properties of the caustic liquor used in the digestion, except to say that typical liquors for Bayer process digestion are used. The short residence time for digestion disclosed in Fulford et al allows the removal of the already small amount of quartz in the bauxite to be minimised and it also allows for solid/liquid separation before significant quantities of sodium alumino-silicate solids have formed in the digestion step.
U.S. Pat. No. 4,614,641 in the name of Grubbs, assigned to Aluminum Company of America, describes a digestion process for gibbsitic bauxites. The bauxite is ground and separated into a coarse fraction and a fine fraction. The fine fraction is subjected to a low temperature digestion at 80 to 120.degree. C. to produce a digestion solution containing some silica (greater than 1.8 g/l) but a significant proportion of the silica in the fine bauxite remains unconverted and unextracted. The product digestion solution also has a high alumina content (near saturation) and a caustic concentration of more than 240 g/l (as Na.sub.2 CO.sub.3).
The coarse bauxite fraction is digested under higher temperatures and pressures. The mixture of solids and liquor resulting from digestion of the coarse fraction contains desilication product, which evidently precipitates in the high pressure digester. This mixture is then contacted with the clarified liquor containing some silica (up to 1.8 g/l) from the low temperature digestion (in either the high pressure digester or the clarifier line), and the overall mixture is subsequently clarified. The fine fraction liquor is desilicated by contacting the fine fraction liquor with the mixture of solid residue and liquor from the coarse fraction digestion. The overall process is based upon the differences between the rates of dissolution of gibbsite and kaolinite as a function of temperature and bauxite residence contact time. It is also noted that the caustic solution used to contact the fine fraction digests substantially all of the gibbsitic alumina, from the fine fraction of the bauxite, and most of the silica remains undigested. Accordingly, this solution must initially have a low alumina content (before contacting the fine fraction of bauxite).
4) Sinter Process
This is an alternative to the Bayer process, commonly used in Chinese and Russian refineries for treatment of very high silica bauxites or non-bauxitic ores such as nepheline. A general description of sinter technology is given in a paper by B. I. Arlyuk and A. I. Pivnev in Light Metals 1992, pages 181 to 195. The ore is calcined with soda and lime under conditions where the products are sodium aluminate and calcium silicate. This mixture is then rapidly leached in water or spent liquor to recover the soda and alumina. During leaching some of the calcium silicate also dissolves leading to very high silica contents in the leach liquor (up to 15 g/l). This liquor is subsequently desilicated prior to precipitation of alumina. The leaching conditions are set to minimise silica dissolution and conversion to desilication product so only a small fraction is taken up.
In summary, in prior art processes for dealing with the problems of high silica feeds to the Bayer process either:
All of the prior art processes suffer from one or more disadvantages. In particular, there is no prior art process which results in the substantial avoidance of economically significant reagent consumption due to silica in Bayer feeds, or which facilitates the substantial deportment of silica in Bayer feeds into separable clean streams from which chemical values can be easily recovered.